1. Field of the Invention
The present invention relates to an optical displacement meter and, more particularly, an improvement in a portion for measuring a wavelength of light under measurement that makes it possible to enhance resolving power and high speed response of a chromatic confocal displacement meter and miniaturize the chromatic confocal displacement meter.
2. Description of the Related Art
A chromatic confocal displacement meter (see JP-A-2008-256679) whose entire configuration is shown in FIG. 1 utilizes the following principle. Specifically, an objective lens 12 exhibiting great chromatic aberration along an optical axis varies in focal length according to a light wavelength (color). The objective lens comes into a focus at a close distance for blue light and a far distance for red light. A confocal point (the position of a focal point of a collimator lens 14 whose chromatic aberration is corrected) that is located opposite to a workpiece 8 under measurement with reference to the objective lens 12 is deemed to be common regardless of a color. When a point source of white light or a wideband point source of light is placed at the confocal point, a color focused on the workpiece 8 changes in a one-to-one correspondence according to the height of the workpiece 8. So long as a spatial filter, such as a pinhole, is provided at the location of the confocal point achieved when light reflected from the workpiece 8 returns and so long as the reflected light is let pass through the spatial filter, light of the color focused on the workpiece can be extracted.
A color (optical wavelength) is specified by use of a spectrometer 26, such as a diffraction grating, provided in a console 20, whereby the height (displacement) of the workpiece 8 exhibiting a one-to-one correspondence with the color can be measured.
In general, there are many cases where light (broadband light) is generally guided to a confocal point of a sensor head 10 by use of an optical fiber 30; where a core of an end face 30A of the optical fiber is taken as a confocal point while likened to a pin hole; and where a diverging ray is given to the collimator lens 14 in many cases.
As illustrated in FIG. 2, of the white color beams radiated on the workpiece 8 after having undergone reflection on the workpiece, light of the color (green light in FIG. 2) focused on the workpiece 8 is selectively collected on the position of the confocal point on the end face 30A of the optical fiber. The light is captured into the core of the optical fiber and guided to the spectrometer 26 byway of an optical fiber coupler 24. In the meantime, light of the other colors is blocked by a circumference of core of the optical fiber end face 30A that is worked as the pin hole, to thus become unable to enter the optical fiber 30.
The spectrometer 26 detects a wavelength of the light returned to an interior of the optical fiber 30, and an output from the spectrometer 26 is input to an electronic circuit 28, where the output is processed.
As illustrated in FIG. 3, a spectrometer portion for specifying a light wavelength (color) of the related-art chromatic confocal displacement meter uses a diffraction grating utilizing a diffracting phenomenon of light, a prism utilizing chromatic dispersion of a refractive index, and an optical element 26A that spatially separates colors from each other, to thus let the separated colors exit. A linear array light-receiving element 26B, such as a C-MOS sensor, a CCD, and a photodiode array, receives light, detects a direction in which the light exited, and specifies a wavelength of the light. In FIG. 3, reference numeral 26C designates a collimator lens whose chromatic aberration for collimating light that has exited from the optical fiber 30 after having undergone reflection on the workpiece has been corrected.
When there is used the linear array light-receiving element 26B, such as that illustrated in FIG. 3, three requirements must be satisfied in order to accomplish high resolving power; namely, (1) a narrow pitch between light receiving elements; (2) a large number of the light receiving elements; and (3) a superior signal-to-noise ratio of each of the light receiving elements.
However, under the constrains that the linear array light receiving elements should be used, it has been difficult to accomplish high speed response and high resolving power as follows.
When a CCD is used, the pitch of the light receiving elements is narrow, the number of light receiving elements is large, and a superior signal-to-noise ratio is achieved. However, since a serial output is produced, exhibition of a high speed response of 1 kHz or more is difficult.
In the meantime, when a C-MOS sensor is used, the pitch of the light receiving elements is narrow, the number of light receiving elements is large, and a parallel output is produced. Although a high speed response of tens of kilohertz or more can be accomplished, it is difficult to accomplish high resolving power and high speed response because of a poor signal-to-noise ratio.
When a photodiode array is used, a superior signal-to-noise ratio is exhibited, and a high speed response is accomplished. However, it is difficult to increase the number of light receiving elements (the number of light receiving elements necessary to accomplish high resolving power is of the order of thousands) by narrowing the pitch of the light receiving elements, and hence difficulty is encountered in accomplishing high resolving power. Moreover, when a signal processing circuit is provided in number equal to the light receiving elements, a problem of an increase in circuit scale also arises.
In the meantime, a noncontact displacement meter of a type other than the chromatic confocal displacement meter includes an electrostatic displacement meter, an optical interference displacement meter, an optical fiber displacement meter, a triangulation displacement meter, a focusing displacement meter for detecting the position of a lens achieved during focusing operation by scanning the objective lens, a confocal displacement meter, and the like.
However, the electrostatic displacement meter can accomplish high speed response and high resolving power but encounters problems of producing a large measurement spot, providing a short working distance, being vulnerable to an inclination, and generating an error because the displacement meter is a nonconductor.
Alternatively, the optical interference displacement meter also exhibits high speed response and high resolving power. However, the displacement meter produces drawbacks of being unable to cope with a step and perform ABS measurement, being vulnerable to an inclination, producing a large measurement spot, and being greatly influenced by surface roughness.
The optical fiber displacement meter exhibits high speed response but encounters drawbacks of requiring calibration for each material, producing a large measurement spot, and providing a short working distance.
The triangulation displacement meter exhibits a comparatively superior response but encounters drawbacks of being difficult to exhibit high speed response with high resolving power and being vulnerable to an inclination.
The focusing displacement meter and the confocal displacement meter produce minute measurement spots and provide long working distances but encounter drawbacks of providing low response and a great heat drift.
As mentioned above, a displacement meter capable of accomplishing high speed response of 100 kHz or more while keeping high resolving power of the order of nanometers (high resolving power in terms of a signal-to-noise ratio rather than display resolving power) is only the electrostatic displacement meter and the optical interference displacement meter. However, the electrostatic displacement meter and the optical interference displacement meter encounter drawbacks of providing a short working distance, being vulnerable to an inclination, and producing a large measurement spot.
Meanwhile, a technique described in connection with JP-B-1-15808, which is not the chromatic confocal displacement meter, is available as one technique for measuring a light wavelength.
As shown in FIG. 4, when a direction of propagation of light is taken as a Z axis and when directions orthogonal to the Z axis are taken as X and Y axes, a light wavelength is measured as follows.
(1) Light under measurement in an arbitrarily polarized state is collimated and caused to propagate in a direction Z.
(2) An orientation of an axis of a polarizer 40 is set to an angle of 45 between the X and Y axes, and light is caused to pass along the axis, whereby linearly polarized light oriented in an angle of 45° between the X and Y axes.
(3) Light is let pass through a wavelength plate 42 having a lead axis aligned to the X axis and a lag axis aligned to the Y axis, whereby elliptically polarized light having a phase difference commensurate with a light wavelength (existing between X and Y polarized waves) is obtained.
(4) Calcite 44 is arranged while inclined in a direction of 45° between the X and Y axes, and the elliptically polarized light is separated into a polarized light component oriented in a direction of an angle of 45° between the X and Y axes and a polarized light component oriented in a direction of an angle of 135° between the X and Y axes. A photodiode (PD) receives respective amounts of light, thereby generating light amount voltage signals A and B.
(5) An analogue circuit (omitted from the drawings) computes a ratio of A to B (A/B) and further subjects a computation result to logarithmic operation, whereby there is obtained a voltage output exhibiting a gentle monotonous change in response to a change in light wavelength within a certain range of a light wavelength.
As shown in FIG. 3, under the method for detecting a light wavelength while separating the wavelength according to directions by means of the optical element 26B, such as a diffraction grating, a distance from the optical element 26A to the linear array light receiving element 26B must be increased in order to enlarge an angular difference between the directions of the light wavelengths as a positional difference. On the contrary, the method shown in FIG. 4 enables easy miniaturization of the displacement meter and is characterized in that higher speed response of signal processing can be accomplished when compared with a case where a slow response linear array light receiving element is used because a high response single photodiode is used.